Electric motors may be controlled by a group of insulated gate bipolar transistors (IGBTs). The IGBTs regulate the flow of current to the stator. The stator is composed of stationary windings in the motor which generate a magnetic field when excited. Current flows through the IGBTs when the IGBTs close (i.e., form a closed circuit), which allows current to flow through the stator windings. The magnetic field generated by the current flowing in the stator interacts with the magnetic field in the rotor. The interaction of the magnetic fields produces a torque on the rotor. The torque generated is proportional to the current flowing in the IGBTs, when the angle between the magnetic fields is held constant. The angle between the magnetic fields is maintained by controlling the switch timing of the IGBTs. By accelerating the switch timing of the IGBTs, the rotor may be accelerated, thus controlling the speed of the motor.
There are two sources of heat in the IGBTs controlling the flow of current to the motor. The first source of heat is due to switching. Non-zero voltages and currents exist at the same time when the switch transitions between states. This voltage and current product produces heat. More heat is produced as the switching frequency increases, because there are more transitions each second. The other source of heat is due to conduction. An IGBT produces a small voltage drop when turned on. The product of the current flowing through the IGBT and the voltage drop across the IGBT is responsible for the heat. More heat is produced for longer periods of time the switch conducts current. Three-phase motors require a pair of switches for each phase. The switches are modulated such that peak currents are constantly shifted between the phases. Therefore, the current delivered to a motor is distributed between three IGBT pairs, so the losses in the form of heat are distributed between six IGBTs.
Before the motor spins, it is in a stationary state referred to as the stall condition. In order to cause rotation in the motor, a magnetic field must be generated in a specific location. The field location is determined by the rotor position. The field is constantly moving in a rotating motor. The field is stationary for a stationary rotor. A specific combination of the six IGBTs corresponds to the placement of the field. As the field spins, the combination of switching IGBTs constantly changes. For the stall condition, only two IGBTs are conducting, so the load is not shared between the other IGBTs. The IGBTs are limited in the amount of heat they can dissipate. Two switches at stall condition cannot dissipate the amount of heat six switches can dissipate while the motor spins. Therefore, under the same switching conditions, the two switches cannot deliver the same amount of current to the motor at stall as the six switches can provide during rotation, thus limiting the stall torque.
Methods have been implemented to avoid failure due to overheating. The first method involves derating the torque at stall. Derating the torque implies lowering the maximum available torque. Because torque is proportional to current, lowering the torque results in a smaller maximum current passing through the conducting IGBTs at stall. The amount of heat produced in an IGBT is proportional to the amount of current passing through the IGBT. Thus, lower current produces less heat, keeping the IGBT within the rated operating capabilities, and failure is avoided. However, because the current is reduced, the torque at stall is reduced, and the full operational range is handicapped based on the nature of the stall condition. Electric drives are specified based on a maximum torque. The maximum torque is needed at stall just as it is needed at the other speeds for which the maximum torque is rated.
A second method to avoid overheating at stall involves duty cycle limiting. The full torque may be applied for some small time period, then a limited torque is applied in a “rest” period. The limited torque requires less current. Less heat is produced during the “rest” period, and the heat generated from the higher current at higher torque is given time to be removed. The result is a series of torque pulses at stall. Overheating can be avoided, but two other problems arise. Torque pulsations may cause damage to mechanical components. In addition, in order to accelerate a load, a specific torque must be held for some minimum time. If the torque pulse time does not meet the minimum time to accelerate the load, the rotor will not begin to spin.